However, this post isn't about whether or not ultrasonography reduces the rate of pneumothorax.Although I am enthusiastic about evidence-based medicine, this question is about as interesting as whether a parachute prevents death after jumping out of a helicopter (Smith 2003).This post is about the changing significance of a post-procedure pneumothorax in the ultrasound era.

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Different causes of post-thoracentesis pneumothorax

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There are three causes of pneumothorax after thoracentesis.The first and most obvious cause is lung laceration by the needle or plastic catheter.This may occur if the operator inserts the needle into the lung.Another way this could conceivably occur may be if the stiff plastic catheter is inserted in the middle of the thorax (rather than over the diaphragm), forcing the lung to re-expand around the catheter.For a fragile lung with bullae, this could tear the lung as it re-inflates (figure below).With ultrasound-guided placement, these problems should be rare.

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The second cause of pneumothorax is air introduced into the thorax through the catheter inadvertently.For example, if the catheter is opened to the air while the patient inspires, air would be pulled into the pleura.Although this does indeed cause a pneumothorax, this pneumothorax is of no clinical significance.It is impossible for this type of pneumothorax to expand or develop into a tension pneumothorax.With good technique, this should be avoidable.

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The third cause is pneumothorax ex vacuo.This occurs if the underlying lung is unable to expand, for example due to bronchial obstruction. In this situation, the pleural effusion originallywas caused by lung collapse, leading to negative pressure in the pleura which gradually pulled in fluid.The primary problem is not the pleural effusion, but rather the fact that the lung cannot expand.Thoracentesis in this situation generates a low intrapleural pressure which transiently opens a tiny hole in the lung to allow air into the pleural space (alleviating the “vacuum”).Pneumothorax ex vacuo only occurs following therapeutic thoracentesis, not diagnostic thoracentesis.

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The emergence of pneumothorax ex vacuo as an important clinical consideration

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With ultrasound guidance and expert technique, lung laceration and introduction of air into the thorax are largely avoidable.For example, the pneumothorax rate following diagnosticthoracentesis should be close to zero.In expert hands, pneumothorax ex vacuoemerges as the most common cause of pneumothorax following therapeutic thoracentesis (Heidecker 2006).Although pneumothorax ex vacuohas always occurred, its relative importance has increased as the other causes of post-procedure pneumothorax have become less common.

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Pneumothorax ex vacuo usually doesn't require any immediate treatment

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Pneumothorax ex vacuo is a benign phenomenon which rarely enlarges or leads to tension pneumothorax.Patients are asymptomatic.This should not be treated with a chest tube, because the primary problem is unexpandible lung and this willnot respond to pleural drainage (Heidecker 2006; Huggins 2010).

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Management focuses on investigating and treating the underlying cause of the lung not expanding.The occurrence of pneumothorax ex vacuo may be helpful because it redirects the diagnostic investigation (for example to evaluate for such entities as trapped lung or endobronchial obstruction).

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Should we limit all thoracenteses to under one liter in efforts to decrease in the rate of pneumothorax ex vacuo?

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Wilcox noted a small increase in the pneumothorax rate when over one liter of fluid was removed at thoracentesis (4.1% vs. 4.9%).Although this difference is not statistically significant, it may relate in part to an increased risk of pneumothorax ex vacuo with larger volume thoracentesis. On this basis, the authors suggested limiting thoracentesis volume to under one liter.

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Pneumothorax ex vacuo is rare, benign, and potentially a useful diagnostic finding. Avoiding pneumothorax ex vacuo could lead the clinician to continue futile efforts to drain the pleural effusion (i.e., with repeat thoracentesis or chest tube; Staes 2009). For a patient with unexpandible lung, these procedures will be ineffective, as the effusion will recur until the underlying atelectasis is resolved.0
The recommendation to limit the volume removed at thoracentesis in order to avoid pneumothorax may reflect a failure to appreciate the significance of pneumothorax ex vacuo. Not all post-procedure pneumothorax is created equal.

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Take-home points

There are many mechanisms of post-thoracentesis pneumothorax. With ultrasound guidance and excellent technique, most can be avoided except for pneumothorax ex vacuo. In expert hands, this becomes the most common cause of pneumothorax after therapeutic thoracentesis.

Pneumothorax ex vacuo is important to recognize as a possible cause of pneumothorax following therapeutic thoracentesis. This is a benign entity which doesn't benefit from chest tube placement and can usually be observed.

Although pneumothorax ex vacuo may possibly be avoided by performing small-volume thoracentesis, its unclear that this is a beneficial approach as it may leave the underlying problem (unexpandible lung) obscured.

This is the first part of a two-part series on thoracentesis. We're only getting warmed up… stay tuned for next week's post.

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Vishnu sharma

A violent cough after or during thoracocentesis can lead to pneumothorax as lung can give way especially if there is underlying parenchymal disease. Trapped lung can also occur if visceral pleura is thickened in which case treatment is decortication.